This proposal requests funds for conducting research on the endocrine control of male sexual behavior. In most male vertebrates, the regulatory mechanism of sexual responsiveness--the "arousal mechanism" of Beach (1956)--is poorly understood. A lack of understanding was particularly apparent for amphibians until the recent experiments using Taricha granulosa, the research subject in this proposal. These experiments provide evidence that initial sexual behavior are dependent on androgens, and that the occurrence of these androgen-dependent behaviors is regulated by a neurohormone. Based on these data, the proposed research program is designed to investigate three aspects of male responsiveness. First, experiments are proposed to determine whether the observed synergism between testosterone and dihydrotestosterone reflects the aromatization of testosterone to estrogen. Secondly, experiments are proposed to identify why arginine vasopressin, a neurohormone, causes a rapid and pronounced increase in male sexual responsiveness, when androgen titers are high. Thirdly, experiments are proposed to investigate the mechanism of action of the effects of the neurohormone on male sexual responsiveness. The biological significance of the proposed research extends beyond that of elucidating the endocrine control of male sexual behaviors in an amphibian. Comparable experiments investigating the role of a neurohormone on androgen-dependent sexual behaviors have never been performed in any vertebrate. Results from these proposed experiments may provide a model of the physiological regulation of male sexual behavior useful to researchers using other species of vertebrates, including mammals.